1. Field of the Invention
The present invention relates generally to an intake and/or exhaust valve timing control system for an internal combustion engine. More particularly, the invention is directed to an intake and/or exhaust valve timing control system which is operable to modify the intake and/or exhaust valve timing quickly, with reduced consumption of working fluid, in response to variation in engine operating conditions.
2. Description of the Prior Art
U.S. Pat. No. 4,535,731 assigned to Alfa Romeo Auto S.p.A. discloses a conventional valve timing control system for an internal combustion engine. In this valve timing control system, a camshaft disposing cams thereon for controlling intake and exhaust valve operations has outer teeth in its end peripheral surface. A timing pulley which is mechanically connected to a crankshaft of the engine through a timing belt has inner teeth in its inside surface. An intermediate cylindrical member or sleeve which includes a hollow portion having inner helical teeth and outer straight teeth is arranged to engage the outer teeth of the camshaft and the inner teeth of the timing pulley. A pressure chamber is defined between the timing pulley and the camshaft to be oriented to the sleeve so that internal pressure thereof acts on a front end surface of the sleeve. The sleeve is always urged toward the pressure chamber by a coil spring. A driving mechanism is provided which includes a hydraulic supply line for directing hydraulic pressure generated by an oil pump to the pressure chamber and a hydraulic drain line for discharging both the hydraulic pressures in the pressure chamber and the hydraulic supply line from a drain port through a relief valve. With this arrangement, a control unit provides a control signal according to engine operating conditions to activate the relief valve to modify a pressure level in the pressure chamber. This modified pressure then acts on the sleeve to be displaced in an axial direction according to a balance between the pressure level of the pressure chamber and a spring force of the coil spring, thereby regulating a phase angle between the timing pulley and the camshaft to advance or retard the valve timing of the intake valves.
However, in the above prior art valve timing control system, a phase angle between the timing pulley and the camshaft is, as mentioned above, changed by the activity of the relief valve arranged in the hydraulic drain line. Therefore, when the relief valve is opened, a large amount of pressurized working fluid supplied from the oil pump is simply discharged to an oil pan of the engine. In other words, a large amount of working fluid (i.e., a lubricating oil stored in the oil pan) circulates between the system and the oil pan. Thus, a supply of lubricating oil for sliding parts of the engine becomes insufficient.
In order to avoid such a drawback, a fixed orifice may be arranged in the hydraulic supply line for restricting an amount of working fluid directed to the pressure chamber. This arrangement, however, also encounters a drawback in that, when it becomes necessary to supply high pressure of working fluid to the pressure chamber, as engine load is increased by acceleration operation by a driver for example, pressure elevation in the pressure chamber is undesirably delayed due to the orifice. Thus, the internal pressure in the pressure chamber required for displacing the sleeve is reduced momentarily, resulting in a response rate for varying the valve timing being delayed.